It's not that there is no way, it's that there isn't a direct route. Sometimes you have to walk in circles until you get offered a compromise with DF modding. All of my suggestions above would give part of the effect you seem to be asking for, even if they're not exactly what you want the finished product to be. For instance, since material emissions can be dependent on the function of a body part, "shoot bolts" could depend on the function of that crossbow-hand. But, like I said, the game treats any body part that can grasp as a hand and you'll see them using that hand for everything else, and wearing mittens on it. Likewise you can use material emissions to spawn pools of bolts and arrows to fire from real crossbows - but the AI wouldn't know how to use them.
Anyway, in mod news, I've been playing around with thunder and lightning secrets. The lightning is debilitating but not exactly dangerous in the lethal sense. It'll knock an opponent out cold for a tick or two every few ticks, and in meta game terms that basically means the fight is over. It's applied as a syndrome-inducing material delivered as a liquid glob. It explodes violently on contact because it has an extremely low boiling point, which makes delivery of the syndrome more likely as it can then be inhaled whereas it otherwise could only take effect on contact. Not ideal, but there's no such thing as lightning or electrical discharge or really a whole lot like it in game terms. There are three methods of delivery, though; as webs sprayed on the ground, kinda dangerous to the caster as sometimes your own tile gets webbed or the webs exploding vapors touch you as well and you end up "zapped", as liquid globs which you can use at range, or by creating pools on the ground anywhere you choose. Usually pools remain liquid, but if a creature steps in them and gets a spatter on their feet/shoes, that spatter will instantly explode, activating the syndrome. The webs and pools make good traps, and the AI can use them even if they're not exactly good at it.
Thunder is still dubious. Using a syndrome like with lightning sounds okay, but so does dust emissions - they're very good at knocking both the caster and target on their bums simultaneously and slamming into obstacles can be lethal, unfortunately. There's not really a way to deliver the knockout dust attack to the target alone, though, besides a material emission giving them an interaction that lets them attempt to attack you with the dust emission, thus making them the epicenter of the destruction and relying on the AI being suicidally stupid. But players don't fall for that, and what's the fun if things can't go south for us too?
I was going to keep them as separate spheres because together they'd be giving the user roughly double the options any of the other combat mages have, which seems unbalanced. And besides, with LIGHTNING, STORM, and THUNDER spheres, if they're both lumped under one then what do I do with the other two?
That's kinda what I mean about walking in circles until you get a compromise. We can have buggy and fun or buggy and unfun, or neither buggy nor fun via doing nothing at all, which is lame.